Heat developable photosensitive materials are well known in the art. For example, heat developable photosensitive materials and their processing techniques are described in SHASHIN KOGAKU NO KISO "HI-GIN-EN SHASHIN-HEN", pp. 242-255 (1982).
Many methods have also been proposed for forming a color image by heat development. For example, methods for forming a color image by coupling between an oxidation product of a developing agent and a coupler are disclosed, e.g., in U.S. Pat. Nos. 3,531,286, 3,761,270, and 4,021,240, Belgian Patent 802,519, and Research Disclosure, issue of Sept., 1975, pp. 31-32.
However, since these conventional heat developable color photosensitive materials are of non-fixing type, a silver halide remains after image formation and causes gradual coloring of the white background when exposed to strong light or preserved for a long time. Besides, these materials generally require a relatively long time for development, and also the color image obtained suffers from high fog and has only a low image density.
To eliminate the above-described disadvantages, there has been proposed a method in which a photosensitive material is imagewise heated to form or release a diffusible dye and the dye is transferred to a mordanted image-receiving material with the aid of a solvent, such as water, as disclosed in U.S. Pat. Nos. 4,500,626, 4,483,914, 4,503,137, and 4,559,290, and JP-A-59-165054 (the term "JP-A" as used herein means an "unexamined published Japanese patent application").
The above-described method still has problems, such as requirement of high developing temperatures and insufficient stability of the photosensitive materials with time. Hence, improved methods have been suggested, in which heat development for dye transfer is conducted in the presence of a base precursor and a trace amount of water thereby to achieve development acceleration, reduction of developing temperature, and simplification of processing, as disclosed in JP-A-59-218443, JP-A-61-238056, and European Patent 210,660A2.
Also with respect to methods for obtaining a positive color image by heat development, various proposals have hitherto been made. For instance, U.S. Pat. No. 4,559,290 teaches a method of using a so-called DRR compound in an oxidized form which is incapable of releasing a dye in combination with a reducing agent or a precursor thereof. According to this image formation system, the reducing agent is oxidized on heat development in correspondence to the amount of exposed silver halide, and the reducing agent remaining non-oxidized reduces the oxidized DRR compound to make it release a diffusible dye. Further, European Patent 220,746A and Kokai Giho 87-6199, Vol. 12, No. 22 disclose heat developable color photosensitive materials using compounds capable of releasing a diffusible dye through the similar mechanism, i.e., compounds having an N-X bond (wherein X represents an oxygen atom, a nitrogen atom, or a sulfur atom) which is reductively split off to release a diffusible dye.
Since a processing solution commonly employed for ordinary photographic processing is not used in heat development systems, most of the above-described heat developable color photosensitive materials contain a base-generating agent making a pH high on processing or contain a combination of silver halide and an oxidizable dye providing compound or a combination of a reducing agent and a reducible dye-providing compound. Therefore, undesired dye release or dye forming reaction tends to proceed within the photosensitive material during preservation, and when those materials having undergone such a chemical change are development-processed, images obtained suffer from background stains arising from the dye released or formed with time.